I am planning to make a hemispherical spangenhelm with the Gjermundbu type dome and Tjele type oculars since similar dome construction to the Gjermundbu helmet is implied by the presence of 8 narrow bands found with the mask from Tjele. I have been trying to figure out the length of the lateral and brow to nape bands, the difficulty being that helmets are oval and not perfectly round in their layout so it isn't as simple as it may sound.

I think I have it mostly figured out but I would still like to hear from the more knowledgeable people on this one. The outer circumference I am going for is 66 cm which should make the inner circumference around 64 cm since I am using 1,5 mm sheet metal.

The best way to calculate the band lengths I have found thus far is to first add and detract the exact same amount from the circumference, the number depending on how squished the ellipse should be, and the results are two numbers that are exactly the same distance from the original circumference. For me those numbers are 58 and 74, since I added and detracted 8 cm from the original amount.

The dimensions of the helmet are thus around 18,4 x 23,5 cm. I then divided the larger circumference by 4 and got 18.5 cm for the two brow to nape bands that go over the top of the dome minus 0.5 cm top spike allowance which rounds it up nicely to 18 cm.

For the two lateral bands I did the same but with the original circumference which gave me 16,5 cm minus the spike allowance which makes for 16 cm long lateral bands.

I used the same process for determining the length of the bands that go under the helmet, except I used the inner circumference of 64 cm, which makes for 17.5 cm long brow to nape bands, and 15.5 cm long lateral bands, but I might have to shorten those bands even more since the inner circumference is from the inside of the brow band, not the inside of the infill panels.

I have made paper mockups using this method and the bands seem to align quite well, but any suggestions from people with hands on experience in making spangenhelms would be very helpful :D